Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized further issues as a result of operation with such systems. As one example, too much blowthrough air to the intake passage via the first exhaust manifold (e.g., scavenge exhaust manifold) may result in reduced engine efficiency. Similarly, too much or too little EGR flow to the intake passage via the first exhaust manifold may also reduce engine efficiency. However, current engine sensors may not be able to accurately measure the EGR flow and blowthrough amount in first exhaust manifold. As a result, a desired amount of EGR flow and/or blowthrough may not be delivered to the intake passage.
In one example, the issues described above may be addressed by a method, comprising: flowing boosted blowthrough air from a plurality of intake valves to a first exhaust manifold coupled to an intake passage via a first set of exhaust valves; and adjusting an amount of opening overlap between the plurality of intake valves and the first set of exhaust valves responsive to a transition from an estimated combustion air-fuel content to a leaner air-fuel content of the blowthrough air on a cylinder to cylinder basis. For example, the transition may include an increase in an oxygen level of the blowthrough air expelled from the first set of exhaust valves determined from an oxygen sensor disposed in either the first exhaust manifold or a runner of each exhaust valve of the first set of exhaust valves. The increase in the oxygen level may be an increase from a lower, first level of oxygen to a higher, second level of oxygen. The total amount of the boosted blowthrough air flowing to the intake passage from the first exhaust manifold during a single engine cycle may then be determined based on the second level of oxygen for each exhaust valve of the first set of exhaust valves. The amount of opening overlap between the plurality of intake valves and the first set of exhaust valves and/or a position of an EGR valve positioned in an EGR passage coupled between the first exhaust manifold and the intake passage may then be adjusted based on the determined total amount of the boosted blowthrough air and a desired amount of blowthrough air. In this way, the desired amount of blowthrough may be delivered to the intake passage, thereby increasing engine efficiency and reducing engine knock.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.